The present invention relates to a composite piston for vibration pumps comprising a driving part made of ferromagnetic material and a pumping part made of plastic and obtained by means of moulding on a metal insert forming the driving part thereof.
Vibration pumps are fundamental components which are very widespread in many applications and in different sectors. In particular these pumps are widely used for feeding boilers of electric household appliances and especially machines for preparing hot drinks by means of infusion with powders containing the ingredients necessary for preparation thereof, such as machines for the preparation of espresso coffee and similar drinks. The growing use of these vibration pumps is accompanied by the need total reliability to be obtained at an increasingly lower cost.
Initial efforts to limit the price of these pumps were directed towards the choice of materials forming the structure of the pumping body and the electromagnetic apparatus and optimisation of their form and size so as to obtain increasingly reliable and economical parts. By way a secondary effort, research aimed at obtaining a greater degree of automation of the production plants also helped reduce the production costs.
At this point, in order to reduce further the production costs, it was necessary to modify certain components which normally are never taken into consideration because they are xe2x80x9capparentlyxe2x80x9d already simple, but which nevertheless may still be the source of defects in the end product, albeit to a very small extent.
One of these components is the piston pump, which hitherto has been made entirely of metallic material by means of mechanical machining.
Hitherto, for the sake of simplicity, it was considered to be particularly economical and logical to construct the piston of a vibration pump as a single metal part.
However, when analysing the methods used for manufacture of an entirely metal piston and the requirements which this piston must satisfy, several facts which apparently had not come to light now emerged:
1. In a traditional vibration pump the piston is immersed in the liquid to be pumped, hence the need for a high magnetic efficiency and high corrosion resistance. Unfortunately these two characteristics are directly opposed since metallic materials, which have an excellent corrosion resistance, are devoid of ferromagnetic properties, while, on the other hand, materials which have excellent ferromagnetic properties and hence a high magnetic efficiency, have a low corrosion resistance. In short, it has been necessary to adopt compromise solutions which, however, are heavily biased either towards a high corrosion resistance, with poor ferromagnetic properties, or towards good ferromagnetic properties, with a low corrosion resistance. Recent research has indeed produced particular materials which provide an excellent balance, ensuring a high corrosion resistance and good ferromagnetic properties, even though this material has been unable to eliminate the occurrence of problems which, in this case, are of a mechanical nature.
2. As an alternative to the mechanical piston immersed in the pumped liquid, it was thought to provide pumps, in which the electromagnetic portion is separate from the pumping portion. However, such a solution implies the need to construct coils containing a large quantity of copper wire, the cost of which is extremely high or, alternatively, to construct pumps with an inferior performance, in particular at high working pressures, which may not always be satisfactory.
3. The traditional vibration pump comprises a piston which must perform various functions:
it has to convert the force, due to the magnetic field of the coil, into movement;
it must provide a hydraulic seal, during the stroke, with the cylinder of the pump body;
it must ensure the dynamic seal of the intake valve; and
it must allow the outflow of liquid into the chamber preceding the pressure chamber.
In order to perform correctly all these functions, the piston must be manufactured to an industrially acceptable standard with very small tolerances in terms of finish, size and geometrical shape. The dimensional tolerances are therefore extremely important and negatively influence the production cost, in the sense that large tolerances result in a higher number of reject components which are not up to standard and smaller tolerances are possible only at the cost of further machining operations which increase the production costs. However, the parts thus manufactured are unable to guarantee fully the overall quality because the critical points are obtained by means of removal of shavings on automatic machine tools which are required to produce millions of parts per year. In this case, any machining imprecision or the presence of burrs or imperfect finishes make it difficult to guarantee 100% quality which can be obtained only by means of costly and rigorous verification procedures during the pre-assembly stage.
In the prior art EP-A-0 288 216 discloses and claims an electrical fluid pump using a wide part, consisting of a ferromagnetic piece working as portion of the pump moved by the magnetic field of a solenoid coil, and a restricted part, consisting of nonmagnetic material (such as plastic or a non magnetic metal), working as a pump piston and inserted into a central bore of the wide part and there fastened by crimping-in of a lip provided at an end of the wide part.
One object of the present invention is that of producing these pistons in a direct and low-cost manner using a simple direct machining process which excludes finishing operations for parts which have already been machined
The above mentioned objects are achieved by a piston according to the present invention comprising a part which is made of ferromagnetic metallic material, limited in extension to the piston zone intended to perform the magnetic driving function, and a part which is made of non-metallic and non-ferromagnetic material and performs the pumping function of the same piston, the metal part performing the magnetic moving function being made of stainless steel possessing good ferromagnetic properties, characterized in that: the part performing the pumping function is made of a plastic material moulded on the metal part and inserted with a part thereof, consisting of a cylindrical blank, in an axial bore.
Apparently in the prior art the use of materials other than metal was discouraged because the mechanical action of a radial seal at high working pressures caused significant wear also in the case of stainless steel, so that the logical doubt arose that any plastic material would have been subject to even greater wear and, in addition, the mechanical assembly of the metal magnetic driving part with the plastic part, as disclosed in EP-A-0 288 216, apparently required costly systems to correct any clearance and ensure the precision and necessary quality control.
However, it was discovered that, if the temperature of the water to be pumped were to remain close to room temperature (15 to 25xc2x0 C.), with maximum temperatures of the piston remaining between 50 and 60xc2x0 C., there would be no particular difficulty in using thermoplastic materials, possibly containing a reinforcing filler, such as, for example polyamides (nylon) reinforced with glass fibres, ground quartz, fumed silica, diatomaceous earth or the like, the piston being obtained by means of moulding of the thermoplastic material onto a stainless-steel insert of the ferromagnetic type. A practical example of a suitable low-cost and commercially freely available thermoplastic material could be nylon 6.6 containing 30% glass fibres.
Obviously, owing to a certain tendency of nylon 6.6 to absorbxe2x80x94albeit in small quantitiesxe2x80x94water, this would limit the life of the piston, although to time periods of a duration such as to render obsolete the device in which a vibration pump containing said piston was installed.
Moreover, if there was an absolute need for total reliability of the pump to be obtained using thermoplastic materials which are not subject to the drawbacks of the abovementioned nylon 6.6, it is possible to find on the market thermoplastic materials, such as oxy-1,4-phenylene-oxy-1,4-phenylene-carbonyl-1,4-phenylene, produced and marketed by Vitrex Plc in Thorton Cleveleys, Lancashire, United Kingdom, under the tradename Peek(trademark), which material is resistance to temperatures much higher than those which can be withstood by nylon 6,6 and has substantially zero water absorption.
The features of the composite piston according to the present invention are illustrated below:
The piston, as already mentioned, is formed by a metal part and by a plastic part.
The metal part is a simple, low-cost, hollow cylinder which is essentially devoid of defects which may adversely affect operation of the pump. Its geometric form and its size are suitable for providing the magnetic driving force. The internal part is shaped so as to form a portion ensuring a secure mechanical fastening to the thermoplastic material which is subsequently moulded on top thereof. In particular:
as in the traditional version, in this version also, the tolerances of the external diameter are ensured by the drawing of steel bars which therefore do not have to be machined using a machine-tool of any kind;
the only dimension which must remain within the tolerance values is the length of the part so as to ensure the hermetic closure of the mould used for injection of the thermoplastic material, this dimension, however, being easy to obtain and to control;
the degree of finish of the internal part of the hole is no longer of any importance because the latter may be lined with thermoplastic resin; on the contrary an inferior finish may favour fixing of the resin to the said wall. In traditional pistons which are made entirely of stainless steel, in addition to the internal hole, the transverse hole for the outflow of liquid into the chamber preceding the pressure chamber requires an optimum burr-free finish because:
an inadequate finish favours surface oxidation and dispersion of the oxides in the pumped liquid (it should be remembered that iron oxides, even though posing absolutely no danger for health, are aesthetically unpleasing owing to their somewhat dark and intense colour and may provide the beverages with an unpleasant after-taste);
possible machining burrs could become detached during operation, coming into contact with the sealing valves and adversely affecting operation thereof.
The plastic part forms the functional structure of the piston, replacing the more critical and delicate parts thereof, which, in the case of an entirely metallic piston, being obtained by means of mechanical machining operations, may have the following defects:
a) In the intake valve seat:
geometrical defects, such as ovalisation or eccentricity;
insufficient degree of finish;
burrs;
metallic machining dust residue.
These defects result in the drawbacks of an imperfect seal, irregular operation and inadequate performance.
During operation, there may also be the risk of imperfect seating of the valve caused by calcareous deposits or detergents which are used when the pump has been left half-empty and inactive for long periods, resulting in sticking of the valve and malfunctioning of the pump.
All of the abovementioned defects are eliminated by a plastic valve seat.
b) In the diameter of the pressure member, defects in the finish and size and tolerance of the diameter result in an important seal, irregular operation and inferior performance.
c) In the chamber preceding the pressure chamber for outflow of the liquid, defects consisting of burrs between the transverse hole and the longitudinal hole and an inadequate finish result in the possibility of the said burrs becoming detached and wedged or trapped inside the valve seat which therefore loses its sealing capacity, while an inferior finish results in potential exposure to oxidation.
All the abovementioned defects are eliminated by a portion made of plastic.
It should be pointed out that, even if oxy-1,4-phenylene-oxy-1,4-phenylene-carbonyl-1,4-phenylene (Peek(trademark)) were to be used as the plastic, which material has a cost, for the same weight, about 38 times greater than that of stainless steel, a considerable saving equal to between 40 and 60% of the cost of a piston which is made entirely of stainless steel is achieved since, if a traditional piston were to be made entirely of stainless steel, considering that it is necessary to have initially a semi-finished steel product with a length equal to that of the entire piston (i.e. magnetic driving part plus compression part), 75 g of stainless steel and high machining costs would be necessary in order to obtain the machined part using a multiple-spindle machine tool. On the other hand, if a composite piston according to the present invention were to be made, not even half the stainless steel semi-finished product in weight would be used, a negligible machined fraction of the steel used would be necessary and the rest of the plastic piston would be obtained by means of a simple moulding operation without further machining since the plastic part, once moulded, is completely finished. At least, the cost of a composite piston according to the present invention, compared to a traditional piston, would be of the order of between 40 and 60% thereof.
If we consider the abovementioned positive aspects, the advantage of providing piston for vibration pumps with a magnetic driving part made of stainless steel and a compression part made of plastic is obvious.